This invention relates to a microporous or ultrafiltration membrane formed from a hydrophobic substrate having a hydrophilic surface including the membrane pore surfaces and to a process for forming such a membrane.
In many applications of filtration technology, it is desirable to utilize a membrane filter which is mechanically strong, is thermally stable, is relatively inert chemically and is insoluble in most organic solvents. Often, it is desirable that the membrane have surface properties which are radically different from, and sometimes incompatible with the bulk properties set forth above. Desirable surface properties include wettability, low protein adsorbing tendency, thromborestivity, controlled ion exchange capacity and controlled surface chemical reactivity.
Conventional methodology presently used to achieve the duality of function of bulk properties which differ from the surface properties is to coat a performed membrane having the desired bulk properties with an oligomer or polymer having the desired surface properties. Typical coating materials include surfactants and water soluble polymers such as polyvinylpyrrolidone. This approach to modifying surface properties is undesirable since the coating is only temporary and exposure to any process fluid, particularly when the substrate having the desired bulk properties is a porous membrane, effects removal of the coating from the porous membrane. Membranes treated in this fashion cannot be steam sterilized, cannot be rewet once dried after being wetted with water and exhibit high extractable levels. These properties are unacceptable in many filtration applications, particularly when processing biological fluids which are to be sterilized or subsequently analyzed.
It also has been proposed to utilize graft polymerization techniques to modify the surface characteristics of a polymeric substrate. Typical examples of graft polymerization are shown for example in U.S. Pat. Nos. 3,253,057; 4,151,225; 4,278,777 and 4,311,573. It is difficult to utilize presently available graft polymerization techniques to modify the surface properties of the porous membrane. This is because it is difficult to modify the entire surface of the membrane including the surfaces within the pores while avoiding pore blockage and while retaining membrane porosity. In U.S. Pat. No. 4,340,482, issued July 20, 1982, it has been proposed to modify the surface of porous membranes formed from hydrophobic fluorine-containing polymers by binding a primary amine such a glycine to the hydrophobic substrate. The primary amine renders the polymer surface hydrophilic and can be utilized as a reactant site to link a polymerizable monomer to the porous membrane thereby to obtain a porous membrane having surface properties corresponding to that of the polymerized monomer. Unfortunately, the modified membranes so-produced exhibit properties which are undesirable for use with certain materials. Thus, the membrane so-produced oftentimes is colored that is, a non-white color, and gives off colored extractable compositions during use. Furthermore, the membranes have a tendency to adsorb proteins from solution and therefore are unacceptable in some applications such as in clinical diagnostic assays. It has been proposed in U.S. Pat. No. 4,618,533 to form a porous membrane having a porous membrane substrate to which is directly coated a cross linked polymer formed from a monomer polymerized with a free radical initiator in situ on the substrate. The resulting composite membrane has essentially the same porous configuration as the porous substrate. It is disclosed that the presence of a polymerization initiator and a crosslinking agent are necessary in order to effect the desired polymerization and crosslinking in situ thereby to obtain the desired porous configuration of the membrane product, i.e., little or no blockage of the pores. It has been found that when the composite membrane produced by the process is wet, that some swelling of the hydrophilic coating occurs which results in some pore blockage during use. In addition, the presence of a low molecular weight, incompletely crosslinked coating can result in small amounts of extractable being released in solution during use of the membrane. Accordingly, it would be desirable to provide a composite membrane having desired bulk and surface properties which retain its porosity even when wet and which has very low levels of extractables.